“The election results are now fully in and Congressman Tierney retains a narrow lead. I congratulate him on his victory,” Tisei said in a statement. “While I have concerns about reported irregularities in some areas of the district, I’ll leave any investigation of those matters to others. I’ll have no further comment on these matters.”

Tierney, who’s from Salem and first took office in 1996, struggled to contain the fallout from a federal criminal case involving his wife and her family. His wife agreed in October 2010 to plead guilty to federal charges she helped her brothers conceal an illegal offshore gambling operation. She was sentenced to 30 days behind bars.

Tierney said he didn’t know the online gambling operation in Antigua was illegal.

Unofficial returns showed fewer than 4,000 votes separating the two candidates out of more than 370,000 cast in the district.

Tisei initially refused to concede the race. His campaign had said it wanted to take a closer look at returns from some communities.

Tisei, a former state senator from Lynnfield who would have been the first Republican to win a U.S. House race in Massachusetts since 1994, accused him of being dishonest with voters in the 6th District, which includes the state’s northern coast.

The GOP poured money into the race to support Tisei, whose campaign said Tierney was insulting the intelligence of voters by claiming he was unaware of the illegal nature of the gambling operation.

Tierney first took office after defeating Republican Peter Torkildsen in 1996. Torkildsen and Peter Blute, who also was defeated that year, were the last Massachusetts Republicans to serve in the U.S. House.